The shells and end-pieces resulting from the chopping-up of irradiated nuclear fuel assemblies (said chopping process has been described especially in patent application EP-A-347 312) have hitherto been placed in the same drum, specific for this type of high level waste, on leaving their respective rinsers. They are then coated (as such) with a cement slurry, said slurry being poured into the drum until it is full. After a safety lid has been welded on, the filled drums are transferred directly to a storage facility.
In order to reduce the definitive storage volume of this waste considerably, it was decided to compact it. A proposed compacting process has been described especially in patent application WO-A-94/16449. The desired procedure is actually as follows:
Compacting this type of waste with a pyrophoric tendency--which in fact consists of metallic materials capable of catching fire and/or exploding--creates genuine technical problems. For obvious safety reasons, the cases containing said waste must be compacted after obligatory drying and saturation with inert gas (internal blanketing), preferably in an inert atmosphere (external blanketing around said cases in the compacting skirt), with obligatory recovery of the gases which escape. These techniques of:
It is pointed out here, in passing, that any other blanketing technique, solid or liquid, is excluded because of the large quantities of inert materials required and because of the incompressibility of said materials. Inert gas is therefore used, the nature of said gas obviously being adapted to the type of pyrophoricity exhibited by the waste to be compacted.
It is also pointed out, for information, that the compacting device used comprises essentially:
The case cracks under the action of the pressure exerted by the compacting means (the characteristics of said case (nature of the constituent material, its thickness . . . ) have been optimized for this purpose of controlled cracking). The internal blanketing gas then escapes through the cracks generated, entraining pyrophoric dust, especially Zircaloy dust. It is imperative that said dust be recovered and carried away to an appropriate treatment unit. At the same time, accumulation of said dust inside the compacting skirt and dissemination thereof in the compacting cell must be avoided at all costs. Said dust is recovered using an aspirating device on top of the compacting skirt.
The aim of the complementary external blanketing is to reinforce the blanketing around the case during compacting, inasmuch as the rate of escape of the internal blanketing gas (initially contained in the case) is difficult to control. Said complementary external blanketing is effected with an injection nozzle and an aspirating pipe arranged above the compacting skirt. This type of complementary external blanketing system has been illustrated schematically in FIG. 2 of patent application WO-A-94/15775.
It has been found, however, that in such a configuration said external blanketing system does not effectively sweep the space situated between the compacting skirt and the assembly comprising the compacting means and the case (it would be more accurate to speak of the assembly comprising the case and the end of the compacting means which has entered said skirt).